Alloy Phase Research Articles

Microstructural Evolution of Quaternary AlCoCrNi High-Entropy Alloys during Heat Treatment.

This study examines the microstructural evolution and mechanical properties of quaternary AlCoCrNi high-entropy alloys after heat treatment at 873 K for 72 and 192 h. The changes in nanostructure and phase transformation based on the heat treatment duration were as follows: B2 dendrite + BCC interdendrite and sigma phases after 72 h; B2 dendrite and interdendritic sigma phases + BCC after 192 h. After annealing, the morphology of the dendritic region shifted from spherical to needle-like, and the interdendritic region transformed from a spinodal-like to a plate-like morphology. Additionally, a phase transformation was observed in the dendritic regions of the annealed alloys at the nano-scale. The presence of the sigma phase in AlCoCrNi high-entropy alloys significantly improved the yield strength to around 1172 MPa; nevertheless, it decreased the compressive strain rapidly to 0.62%.

Microstructural characterization of gadolinium-rich phases in titanium alloys

This study explores the microstructural characteristics of gadolinium (Gd)-rich phases in titanium (Ti) alloys through comprehensive electron microscopy analysis. The Ti alloys were produced using plasma arc melting and subsequently hot-forged. Elaborate material characterization, including scanning electron microscopy, electron backscatter diffraction, and energy dispersive spectroscopy, revealed the formation of round or angular Gd oxides and elongated Gd-rich grains within the alloy. High-magnification transmission electron microscopy and X-ray diffraction confirmed the presence of the FCC-type γ-Gd phase, influenced by the oxygen intake during casting, coexisting with Gd2O3 due to their similar crystal structures. The study also observed internal twins in the Gd grains, potentially delaying the transformation to the stable α-Gd phase. The significant mechanical property differences between the Gd-rich phases and the Ti matrix caused defects at phase interfaces during hot processing, weakening the Gd phase. This work enhances the understanding of Gd phase formation and its implications on the mechanical properties of Ti–Gd alloys.

Investigating the corrosion behavior of as‐cast Mg‐Zn‐Ca alloys with the same Zn/Ca atomic ratio by in situ observation

AbstractThe alloying contents with the same Zn/Ca atomic ratio of 0.13 on the corrosion behavior of Mg‐Zn‐Ca (ZX) alloys were investigated. The second phases in ZX10, ZX20, ZX30, and ZX51 alloys were identical with a majority of Mg2Ca and very few Ca2Mg6Zn3 and their distribution changed from isolated to continuous state in ZX30 and ZX51. ZX10 had the lowest corrosion rate of 0.55 mm/year after 168 h immersion in simulated body fluid solution due to the smallest number of sites of galvanic corrosion. The corrosion rate of ZX30 alloy was the highest (0.72 mm/year), while though ZX51 alloy had the most Mg2Ca, its corrosion rate was reduced to 0.69 mm/year, resulting from the small size and thin Mg2Ca. In situ microstructure observation demonstrated the high fraction Mg2Ca phase and continuous Mg2Ca with bulky size were the main reason for the fast corrosion rate.

Sandwich-structured In–Ga alloy phase change thermal pad with low thermal resistance and leakage prevention

Sandwich-structured In–Ga alloy phase change thermal pad with low thermal resistance and leakage prevention

Effect of post-deposition heat treatments on the microstructure and tensile properties of wire-fed electron beam directed energy deposition GH4169 Ni-based alloy

The microstructure and tensile properties of GH4169 alloy fabricated by wire-fed electron beam directed energy deposition (EB-DED) were investigated in both as-deposited (As-Dep) and homogenization plus double aging heat treatment (HA) condition. The results show that thermal cycling caused an aging effect in the As-Dep samples, resulting in the precipitation of the γ’/γ” phases in close proximity to the Laves phase. The dissolution behavior of Laves phase in GH4169 alloy at various homogenization heat treatment temperatures and holding times was studied. Based on experimental results and Johnson-Mehl-Avramie-Kolmogorov (JMAK) analysis, the dissolution activation energy of Laves phase was calculated as 218.2 kJ/mol. The heat treatment regime utilized in HA treatment consisted of 1150 °C × 60 min/WC + 720 °C × 8 h/FC to 620 °C × 8 h/WC. After HA treatment, most of the Laves phases were dissolved and the γ’/γ” phases were precipitated. The mechanical properties of HA samples are superior to those of As-Dep samples. The morphology, distribution and evolutionary mechanism of the Laves phase, γ’/γ” phases are discussed and analyzed.

Machine learning assisted study of phase and properties in cobalt-free AlCrxCuFeNi2 high-entropy alloys

The application of machine learning (ML) accelerates the discovery of new materials, the prediction and optimization of properties, and the analysis of the correlation between structure and properties. The design, characterization, and mechanical properties of the novel AlCrxCuFeNi2 cobalt-free high-entropy alloys (HEAs) were investigated in this study. Through the experimental validation combined with the predictive K-nearest neighbor model (KNN) and the extreme gradient boosting model (XGBoost), the influence of Cr content on phase evolution and mechanical behavior were analyzed comprehensively. The results indicate that the optimized KNN and XGBoost models can achieve the 10-fold average cross validation (CV) accuracies of 0.761 and 0.836, respectively. Eight components of AlCrxCuFeNi2 HEAs are selected for experimental verification. The experimental results shows that the microstructures of the eight alloys are composed of FCC + BCC phases, which is agreement with the prediction. With the increase of Cr content, the dendrite morphology becomes finer and the lattice constants increase. The addition of Cr results in the hardness values from 332.4 HV to 447.7 HV and the deviation from the predicted ones is between 5.4 HV and 37.3 HV. It is worth noting that wear resistance is consistent with the change in hardness values, the specific wear rate decreases from 3.152 × 10−4 mm3/Nm to 6.357 × 10−5 mm3/Nm, and when x = 2.0, the wear resistance is the best. This research underscores the efficacy of ML in expediting the discovery and optimization of HEAs, offering valuable insights for promoting materials design and engineering applications.

Phase-Field Simulation of Precipitation and Grain Boundary Segregation in Fe-Cr-Al Alloys under Irradiation.

A phase-field model for the precipitation of Fe-Cr-Al alloy is established incorporating grain boundary (GB) effects and irradiation-accelerated diffusion. The radiation source and grain boundary effect are incorporated to broaden the applicability of the Fe-Cr-Al precipitated phase-field model. The model is firstly employed to simulate the precipitation of the Cr-rich α' phase in a single-crystal alloy. The precipitation rate and the size distribution of the precipitated phase were analyzed. Subsequently, the model is utilized to simulate segregation at GBs in a double-crystal system, analyzing the enrichment of Cr and depletion of Al near these boundaries. The simulation results are consistent with experimental observations reported in the references. Finally, the model is applied to simulate the precipitation in a polycrystalline Fe-Cr-Al system. The simulated results revealed that the presence of GBs induces the formation of localized regions with enhanced Cr and Al content as well as depleted zones adjacent to these boundaries. GBs also diminish both the quantity and precipitation rate of the formed phase within the grains.

MgAl hydrotalcite supported ultrafine PdNi bimetallic catalyst for the dehydrogenation of dodecahydro-N-ethylcarbazole

Liquid organic hydrogen storage is a promising way of storing hydrogen. The development of efficient and low-cost dehydrogenation catalysts is still required to advance the commercialization of N-ethylcarbazole/dodecahydro-N-ethylcarbazole (NEC/H12-NEC) hydrogen storage and transport system.In this paper, PdNi bimetallic catalyst (Pd3Ni1/LDHs) supported on MgAl hydrotalcite was prepared by NaBH4 and ultrasound-assisted stepwise reduction. Ultrasonic cavitation could excite the hydroxyl hydrogen of hydrotalcite in the form of hydrogen radicals to reduce PdCl42− to Pd nanoparticles (NPs), and also promoted the formation of PdNi alloys as well as the dispersion of NPs. In the Pd3Ni1/LDHs catalyst, the average particle size of NPs was 1.98 nm, suggesting an excellent dispersion of Pd, Ni, and PdNi NPs. The electronic effects of Ni and Pd significantly improved the catalytic efficiency of Pd3Ni1/LDHs in H12-NEC dehydrogenation, resulting in a hydrogen release rate of 98.4 % after 6 h at 180 °C, surpassing that of Pd/LDHs. It can be considered that the dehydrogenation performance of Pd3Ni1/LDHs for H12-NEC is related to the alloy phase, electronic structure and NPs size.

Computational thermodynamics-guided alloy design and phase stability in CoCrFeMnNi-based medium-and high-entropy alloys: An experimental-theoretical study

A computational thermodynamics approach has been employed to design CoCrFeMnNi-based medium- and high-entropy alloys (M/HEAs) with systematically varied compositions (Co((80-X)/2)Cr((80-X)/2)FeXMn10Ni10 with x = 30, 40, and 50 at.%) and phase stability. Since the formation of sigma phase, usually brittle and undesirable, is a common concern, when this class of alloys is subjected to elevated temperatures (600–1000 °C), predicting its formation becomes essential. Thus, its formation and the phase equilibria were studied using the CALPHAD method, and two empirical methods, namely, valence electron concentration (VEC) and paired sigma-forming element (PSFE). Isothermal aging treatments at 900–1100 °C for 20 h were performed, since CALPHAD and VEC/PSFE predictions diverged. Both prediction methods were compared with experimental characterization by a combination of scanning electron microscopy and high-energy synchrotron X-ray diffraction. The predictions from the VEC/PSFE and CALPHAD calculations (depending on the database used) were shown to be quite accurate.

Thermodynamic and elastic properties of laves phase AB2-based alloys and their hydrides: A density functional theory (DFT) study

A first-principles method based on the density functional theory (DFT) was employed in conjunction with quasi-harmonic Debye model to investigate the structural, elastic, and thermodynamic properties of a series of AB2-based Laves phases alloys and their hydrides. Simulation results revealed that the studied materials possess the Laves phase structure with lattice parameters comparable to experimental findings. For the first time, to the best of our knowledge, mixing entropy determined using Debye model was used to classify alloys into (Low- Medium-High Entropy Alloys) LEA, MEA and HEA categories rather than the commonly used ideal solution model, which is often inaccurate and ignores the impact of temperature. Lattice analyses of the alloy materials indicated that cell volume increases with the addition of elements, while the enthalpy of hydride formation indicates that hydrogen absorption in these alloys is exothermic and that the 3.00 H/F.U configuration is energetically stable. The alloys and their hydrides are metallic with no band gap at the Fermi level. The thermodynamic properties were studied using the quasi-harmonic Debye model and it was found that Bulk modulus decreases with increasing volume, and the hydrides possess lower bulk modulus compared to their metallic counterparts. Moreover, Debye temperature decreases with the gradual addition of elements, indicating weaker chemical bonds in ternary and other alloys. All hydrides have lower Debye temperature than their parent alloy materials. Finally, The alloys are classified into low-, medium-, and high-entropy alloys based on mixing entropy calculated using the Debye model. TiMn2 is classified as a low-entropy alloy, ZrMn2, Ti0.5Zr0.5Mn2, and Ti0.5Zr0.5MnFe as medium-entropy alloys, and Ti0.5Zr0.5(MnCr)2, Ti0.5Zr0.5Mn2/3Fe2/3Cr2/3, and Ti0.5Zr0.5Mn0.5Fe0.5Cr0.5Ni0.5 as high-entropy alloys.

Phase Transformation of AlV55 Alloy at High Temperature

Vanadium–aluminum alloy is an important intermediate alloy for preparing aviation grade titanium alloys, and its product quality directly affects the finished product quality of titanium alloys. In this study, focusing on the problems of high powder content (19.8%) and low product yield in AlV55 alloy products, we conduct research on alloy quality control technology and implement a vanadium–aluminum alloy cooling crystallization control process. The research results indicate that there are three phases in AlV55 alloy, namely Al8V5, AlV, and Al2V3 phases. As the temperature decreases, the AlV phase gradually decomposes into Al8V5 phase and Al2V3 phase, and the proportion of Al8V5 phase is positively correlated with the fineness. Rapid cooling can reduce the formation of Al8V5 phase. The experimental results show that high-temperature water quenching can increase the proportion of vanadium–aluminum solid solution phase in the alloy from 19.03% to 31.76%, and reduce the fine powder rate to 13.2%, providing important product quality control means and technical support for the production of vanadium–aluminum alloys.

Investigation of the structural, Electronic and magnetic properties of Mn2PdSn Heusler alloy under hydrostatic pressure

The pressure effect on the structural, electronic and magnetic properties of Mn2PdSn Heusler alloy was investigated by the first-principles full-potential linearized augmented plane wave (FP-LAPW) method based on density functional theory (DFT). The exchange and correlation function has been chosen using generalized gradient approximation (GGA) within the parameterization of Perdew-burke-ernzerhof. The lattice constant, electronic density of state (DOS), and magnetic properties such as total magnetic moment were obtained under pressure. It was determined that the calculated lattice parameters are in good agreement with the literature. The Mn2PdSn is stable in the Hg2CuTi-type structure. Furthermore, we expect an extraordinary occurrence of pressure-induced metallic ferrimagnetism to half-metallic ferromagnetism transition in the cubic phase of Mn2PdSn alloy at hydrostatic pressures of 10 GPa.

On the formation trajectory of diverse corrosion morphologies associated with T phases in an Al-Cu-Li alloy

Four different types of localized corrosion morphologies associated with Al20Cu2Mn3 dispersoids (T phases) in an AA2195 alloy were traced using atom-resolved characterizations and quasi-in-situ experiments. At the atomic scale, we demonstrated that the rod-shaped T dispersoid is homogeneous within the grain but enriched with Cu on the side surfaces, leading to a depletion of T1 precipitates nearby and an abundance near the tips. This plays a significant role in the initiation of galvanic corrosion between T dispersoids and the matrix. Combined with heterogeneous nucleation, the subsequent redeposition of noble metal clusters results in four typical corrosion morphologies.

Microscopic damage in eutectic SnPb alloy: First-principles calculations and experiments

This study aims to comprehensively elucidate the microscopic origins of damage in eutectic SnPb alloy through atomic-scale investigations, a crucial endeavor for enhancing the alloy’s properties and furthering the development of Pb-free solders. First-principles calculations based on density functional theory (DFT) were employed to examine the mechanical constants and defect formation energies of the Pb phase, the β-Sn phase, and potential Sn/Pb eutectic phases in the SnPb alloy. The Voigt-Reuss-Hill approximation was utilized to derive the mechanical parameters of the elastically stable structure in SnPb alloy, facilitating the prediction of its mechanical properties. The defect formation energy calculations revealed a higher difficulty in forming vacancy defects in the β-Sn phase compared to the Pb phase. This discrepancy can be attributed to the stronger interactions between Sn atoms, resulting in the formation of more stable Sn-Sn bonds, thereby impeding vacancy generation in the β-Sn phase. Furthermore, the Pb phase exhibited a tendency for Sn atoms to occupy octahedral interstitial sites, whereas in the β-Sn phase, Pb atoms tended to displace neighboring Sn atoms along the [110] orientation into adjacent interstitial sites and occupy their lattice positions. Experimental observations complemented these calculation findings, demonstrating significant Sn element presence, up to 27.22 at%, in the Pb-rich phase, with notable microvoid damage primarily concentrated within this region. This observation aligns with the conclusions drawn from first-principles calculations and unveils the microscopic origins of damage in eutectic SnPb alloys.

Microstructure and mechanical properties of CoCrNiVAlx multi-principal component alloy

Multi-principal component alloy has been proved to form disordered solid solution in most cases. However, multi-principal component alloy with single phase solid solution has a few disadvantages in mechanical properties, such as low strength or poor plasticity. Aiming at CoCrNi-based multi-principal component alloy with low strength, this paper proposed a method to enhance the multi-principal component alloy through composition design. CoCrNiVAlx multi-principal component alloy was melted by non-consumable vacuum arc melting, and the content of σ phase in the multi-principal component alloy was changed by adjusting the content of Al element. With the addition of Al element, the fcc disordered phase in the multi-principal component alloy gradually went to the ordered L12 phase with the same structure. When the amount of Al was 1.5 wt%, the compressive strength reached 2347 MPa and the hardness reached 760.8HV, which were 30% and 20% higher than the compressive strength (1828 MPa) and hardness (619.3HV) of CoCrNiV multi-principal component alloy, respectively. With the further increase of Al element to 2.0 wt%, the σ phase in the multi-principal component alloy increased further, resulting in the simultaneous decrease of strength and plasticity. The strengthening mechanism of multi-principal component alloy was analyzed, and the strengthening action of L12 phase was confirmed. The results showed that the strengthening of multi-principal component alloy was mainly provided by L12 phase strengthening (1229.55 MPa, 52.39%) and grain boundary strengthening (376 MPa, 16.02%). This paper provided a probable strategy to increase the strength of the multi-principal component alloy, which opened up a new idea for the composition design of multi-principal component alloy.

Interface structure of α-Mg/14H-LPSO: First-principles prediction and experimental study

The interfacial structure of the α-Mg/14H-LPSO phase in rare earth-including magnesium alloy was investigated via high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging and first-principles calculations of density-functional theory. Eleven possible interfacial models were constructed according to the different terminations of the LPSO phase, and the corresponding interfacial energies were calculated, from which the four most stable structures (Ter1-MgY-hollow, Ter2-Zn-hollow, Ter3-MgYII-hollow and Ter4-Mg-bridge) were obtained. The interfacial phase diagrams related to the Y chemical potentials were obtained from the calculations, and the most stable interfacial structure was evaluated. Ter1-MgY-hollow and Ter2-Zn-hollow have the lowest interfacial energies in the range of −0.7 eV < ΔμY < −0.6 eV, where fluctuating change of state is the minimized and the interface is the most stable. The separation work of the two models was calculated to predict the bonding strength of the structures at both ends of the interface. The calculation results show that the maximum interfacial separation work is 1.45 J/m2 for the interface model of α-Mg and 14H-LPSO phase structure with Ter2-Zn-hollow termination.

Effective response and microstructure evolution for shape memory alloy laminated composites

A model for the macroscopic mechanical behavior of rank-1 laminates including two shape memory alloy (SMA) phases is presented. The model expresses the general behavior of the composite with phases undergoing rate-independent elastic and inelastic deformations. Homogenization techniques (including the rank-1 laminate model) are used to establish the effective behavior of the SMA laminated composite (SLC) based on the information about the mechanical response of the individual phases and their volume concentrations. A stress-control algorithm is put forward to implement the model. With the aid of the stress-control algorithm, an implicit expression for the effective tangent stiffness and an evolution equation for the effective inelastic strain are derived. Results are compared with the outcomes of an FE-based computational homogenization and a very good agreement is seen. By using a constitutive model with internal variables for the dense SMA, the overall response of the SLC under different mechanical loadings is evaluated. The effective response of the SLC for various volume concentrations of the phases is assessed and exclusive comparisons are illustrated. Furthermore, the influence of different temperatures on the effective superelastic behavior of the SLCs is studied. The findings have implications for the analysis and the design of more complex shape-memory-alloy laminated composites for high-end applications.

Exploring silicon nanoparticles and nanographite-based anodes for lithium-ion batteries

This study investigates the performance of silicon nanoparticles (Si NPs) and silicon nanographite (SiNG) composite-based anodes for lithium-ion batteries (LiBs). Si offers a promising alternative to traditional graphite anodes due to its higher theoretical capacity, despite encountering challenges such as volume expansion, pulverization, and the formation of a solid electrolyte interface (SEI) during lithiation. SiNPs anode exhibited initial specific capacities of 1568.9 mAh/g, decreasing to 1137.6 mAh/g after 100th cycles, with stable Li–Si alloy phases and high Coulombic efficiency (100.48%). It also showed good rate capability, retaining 1191.3 mAh/g at 8400 mA g−1 (2.82C), attributed to its carbon matrix structure. EIS indicated charge transfer with RB of 3.9 Ω/cm−2 and RCT of 11.4 Ω/cm−2. Contrastingly, SiNG composite anode had an initial capacity of 1780.7 mAh/g, decreasing to 1297.5 mAh/g after 100 cycles. Its composite structure provided cycling stability, with relatively stable capacities after 50 cycles. It exhibited good rate capability (1191.3 mAh/g at 8399.9 mA g−1), attributed to its carbon matrix structure. Electrochemical impedance spectroscopy showed higher resistances for RB of 4.2 Ω/cm−2 and RCT of 15.6 Ω/cm−2 compared to SiNPs anode. These findings suggest avenues for improving energy storage devices by selecting and designing suitable anode materials.

Laser cladding Ni60 @ WC/ Cu encapsulated rough MoS2 self-lubricating wear resistant composite coating and ultrasound-assisted optimization

This research aims to broaden the scope of self-lubricating wear-resistant coatings for applications in diverse industries such as automotive, metallurgy, power, and aerospace. Employing laser cladding technology, we successfully fabricated high-performance self-lubricating ceramic composite coatings. A comprehensive investigation was conducted to understand the inhibitory effect of Cu on the thermal decomposition of MoS2, and the study systematically explored the relationship between powder composition, coating structure, and organizational properties. The mechanisms behind friction reduction and wear resistance were unveiled, shedding light on the formation of the MoS2 self-lubricating protective film. Research findings reveal that during the laser cladding process, Cu and Ni undergo solid solution, resulting in the formation of the Cu–Ni alloy phase and crystal refinement. The MoS2 aggregation area exhibits a fine dendritic structure, while the dispersion area showcases coarse dendritic and cellular crystals. The addition of Cu and MoS2 influences the content of the MxCy phase and the thermal decomposition of MoS2. The incorporation of Cu increases the average coating hardness, whereas MoS2 addition decreases it; nevertheless, the Cu/MoS2 coating hardness is enhanced by at least 6.4 %. Cu significantly improves the coating's wear resistance, with a relatively smaller impact on friction reduction. MoS2 functions as a friction-reducing phase during wear, effectively preventing the peeling of hard phases and reducing the friction coefficient. Cu is uniformly distributed in the coating, experiencing solid solution strengthening, reducing adhesive region areas, and minimizing wear debris generation. MoS2, although unevenly distributed, forms intermittent lubricating films on the surface. The lubricating film of the Cu/MoS2 coating remains stable, preventing mutual contact of the friction surface and concurrently reducing the friction coefficient and wear amount. While the study successfully prepared a self-lubricating ceramic coating with excellent wear resistance, some surface quality defects persist. Further optimization of the preparation method was achieved through ultrasound-assisted technology.

Influence of step-by-step cerium-based conversion and polydopamine duplex coating on stir cast Al[sbnd]Cu[sbnd]Mg alloys mitigating saltwater corrosion

Intergranular corrosion (IGC) originates from secondary phases along the grain boundaries in AlCuMg-based alloys can dramatically compromises the tensile properties for structural applications. Insofar cerium-based conversion coating (CeCC) has been exploited to provide a cathodic barrier and mitigate intergranular crack propagation. However, CeCC-based coating deposited on secondary phases in AlCuMg alloys limits its potential use to improve corrosion resistance in the chlorinated environment. Herein, mussel-inspired polydopamine (PDA) coating has been explored on CeCC-treated AlCuMg alloy, providing both anodic and cathodic protection and significantly improving the corrosion resistance in 0.1 M NaCl solution. The micro-morphology of CeCC and CeCC/PDA duplex coating was investigated using X-ray diffraction, field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), two and three-dimensional surface topography, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) reveal the formation of aluminum and/or cerium oxide/hydroxide-based complex coating [Ce2O3/CeO2/Al2O3/Al(OH)3] on the Cu-rich secondary phases [Al2Cu, Al2Cu(Mg), and Al5Si(CuMg)] providing cathodic protection. However, additional PDA treatment for 12 h provided uniform surface coverage of PDA particles along the grain boundaries as well as α-Al matrix owing to the formation of hydrogen bonding with hydroxy species [Al(OH)3/Ce(OH)4] and catechol and amine functional groups present in PDA. Potentiodynamic polarization (PDP) illustrates that the CeCC/PDA specimens can successfully retard the corrosion of AlCuMg alloy. In particular, CeCC-60 s/PDA specimen demonstrated the lowest corrosion current density (Icorr ∼ 0.16 μA/cm2), and the highest polarization resistance (Rp ∼ 119 kΩ·cm2) during electrochemical corrosion in 0.1 M NaCl solution. The corroded surface discloses accelerated IGC was evident on bare and CeCC specimens. On the contrary, CeCC-60 s/PDA specimen demonstrated a significant reduction in IGC and drastically improved corrosion rate.